The present invention is related to the field of nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) sensing tools deployed in down hole well logging and monitoring while drilling environments. More specifically, the invention is related to a NMR well logging and monitoring while drilling tool that comprises an active antenna signal conditioning circuitry for providing multi-frequency measurements and respective suppression of noise.
NMR well logging instruments are utilized for determining properties of earth formations including: the fractional volume of pore space, the fractional volume of mobile fluid filling the pore space and other petrophysical parameters. An NMR well logging instrument typically contains a permanent magnet to produce a static magnetic field in adjacent earth formations. The NMR well logging instrument typically includes a transmitting antenna assembly positioned near the magnet. The transmitting antenna assembly is shaped so that a pulse of radio frequency (RF) power irradiated by the antenna assembly induces a RF magnetic field in the adjacent earth formation. In the investigated volume of the surrounding borehole formation the induced RF magnetic field is generally orthogonal to the static magnetic field, thereby creating appropriate conditions for formation NMR excitation.
Following the RF antenna pulse, voltages representative of NMR precess in the formation are induced in the receiving antenna. In particular, these voltages represent precession rotation of hydrogen or some other nuclei spin axis about the static magnetic field produced by the NMR well logging tool. NMR tool designs typically use the same antenna for transmitting and receiving along with electronic modules for protecting a receiver small signal circuitry from potentially damaging high voltage conditions while transmitting.
There are various known NMR well logging instruments proposed and/or implemented for measuring NMR properties of substances, in particular, the properties of earth formations. One type of NMR instrument is described in U.S. Pat. No. 4,710,713 by Taicher et al. Another type of NMR instrument is described in U.S. Pat. No. 4,350,955 by Jackson et al. Both of these NMR instruments represent early designs of well logging NMR instruments with the main focus on the magnet assembly.
As commonly applied in NMR tools, a primary burst-type pulse of alternating magnetic field with radio frequency (xe2x88x92xe2x80x9cthe RF pulsexe2x80x9d) is irradiated by a transmitter antenna to be applied to a test sample, or in the case of a logging down hole apparatus, to be applied to the formation adjacent the well bore. The test sample or formation responsexe2x80x94a magnetic field due to NMR nuclei spin axis precessionxe2x80x94is typically measured and sampled at a later time, i.e., after the primary RF field has been removed. Typically, the primary transmitter RF pulse and the formation response occur at the same frequency. In this case there is no need for compensation in hardware or in post-processing to correct errors introduced by the primary field that might couple into the receiving antenna.
An xe2x80x9celectronicxe2x80x9d dynamic range of typical NMR operations, that is, the ratio of the primary RF excitation pulse voltage applied to the transmitter antenna to the secondary response voltage induced in the receiver antenna can be of the order of 240 dB or more. The combination of these two factorsxe2x80x94same transmit and receive signal frequencies and such an enormous disparity between the excitation and response voltages creates well known receiver circuitry protection and signal-to-noise ratio design problems. Those who are skilled in the art would recognize that just mentioned problems could appear in both combined (single) and individual transmitter/receiver antenna NMR instrument configurations.
A single or combined transmitter/receiver antenna is generally utilized to provide better stability of the antenna transfer function with respect to excitation and response voltages and, in some applications, for design simplification. Typically, an antenna is provided as a tuned parallel resonant inductance/capacitance (LC) tank circuit with a high electrical quality value (Q), where the inductor L is magnetically coupled to an adjoining sample or formation that is being excited.
The RF pulse induced by the current flowing through the inductor L in the transmit mode excites the nuclei in the adjoining sample or formation. Driving the LC tank by the electronic module acting as a voltage source allows this inductor current magnitude to be almost independent from the tank tuning conditions. However, ability to tune a high electrical quality Q tank exactly to its resonance frequency enables a transmitter design with significantly less power consumption compared to a non-tuned antenna approach. This occurs due to a known fact that the output current drained from the driving electronics will be in Q times less than a resonance tank current itself.
In accordance with the principal of reciprocity, in the receive mode the magnetic field generated by the excited nuclei induces an electrical voltage across the inductor, L of the LC tank and thereby energizing it. This voltage becomes effectively amplified Q times if measured across the entire tank, i.e., across tank capacitor C. Useful additional features of a high Q resonant tank is the narrow filtering capability resulting both in filtering the induced signal and suppressing intrinsic Johnson noise outside of the antenna""s pass-band. A combined transmitter/receiver antenna circuit, however, exhibits the most severe xe2x80x9celectronicxe2x80x9d dynamic range design problems. For example, in the NTMR arrangement, a transmitter driver voltage output pulse is applied in the range of 1,000 volts peak to the combined antenna, however, the response voltage induced by the formation in the combined antenna is only a few tenths of a nanovolt. If such a transmitter voltage is directly applied to the small signal receiver circuitry without an adequate protection, the circuitry would be saturated during the transmission pulse and then might require an inordinately long xe2x80x9cdeadxe2x80x9d time for recovering after the primary RF pulse has been removed. Furthermore, if not properly protected, this relatively large transmission voltage could irreversibly damage the receiver circuitry. In order to avoid the potential damage a protection element or attenuator is placed between the antenna and the receiver circuitry""s amplifier.
The attenuator has been designed to produce a high band pass signal attenuation during the RF field transmission, however, it might undesirably shrink the relatively miniscule response voltages in the receive mode, as well. Moreover, if the attenuator is somewhat noisy, its parasitic signals could be coupled back into the resonant tank circuit and their energy maybe enough to induce undesirable tank self-oscillations. These oscillations, being within the same frequency range as the input signal, would interfere with the acquired response from the formation apparently reducing the signal-to-noise ratio (SNR). To avoid developing excessive noise in the receive mode, a typical attenuator design would assume its disabling or bypassing during reception.
After RF pulse transmission is completed the energy that has been stored in the tank circuitry shall be quickly removed. Conventionally, this has been done by dumping an electronic module by active loading (or shunting) the resonant tank. That is, the electromagnetic energy stored in the tank after transmission is typically converted into the heat and dissipated. For those who are skilled in the art it should be seen that the quickest energy dissipation from the resonant tank happens when this module has been actively loaded with a resistor equal to the tank""s critical impedance.
To further protect the receiver circuitry, the attenuator remains enabled until the dumped tank voltage drops down below a safe level specified by manufacturers of the components from which this circuit is built.
In order to deal with noise coming from other low voltage control circuits, typical NMR antenna circuit designs provide a separate receiver dump circuit that can also suppress the tank resonant qualities. The receiver dump is enabled after completing the energy removal from the tank below safe level, but before switching off the attenuator. It is disabled right before arrival of a respective formation response to the antenna.
Alternatively, separate transmitter and receiver antennas have been used to alleviate the dynamic range severity. The separate transmitter and receiver circuitry solves the problem only partially because the RF transmission pulse is applied directly to the transmitter antenna and is not applied directly to the receiver circuitry. In reality, a practical assembly will always contain some undesirable electromagnetic coupling between these two antennas: it could be capacitive or magnetic or, which happens more often, of both kinds. As the result of this unavoidable coupling the induced voltage in the receiver antenna associated with the transmission pulse can still be sufficiently large to saturate or even damage electronic components in the receiver circuitry. Thus, even in case of separate transmitter and receiver antennas there is also a need for hardware to protect the receiver electronics while the tool is in a transmit mode.
Operating conditions and requirements are different for down hole logging and monitoring applications, as opposed to laboratory conditions. Moreover, there is a significant difference between the capabilities of common laboratory NMR spectrometers and downhole NMR spectrometers. Laboratory NMR spectrometers generally utilize a primary magnetic burst of a single RF frequency for an entire set of tests. The laboratory measurements are generally more stable because of stable laboratory temperature and pressure environment. Thus, the tool electronic attenuators can be set and tuned to a single frequency. If a frequency change becomes necessary, for instance, due to an unsatisfactory test sample response, the laboratory NMR antenna and attenuator can be easily re-tuned to another frequency and the NMR measurement repeated. Laboratory NMR spectrometers usually utilize significantly higher operating frequencies than down hole NMR spectrometers that in some cases this may help to simplify laboratory re-tuning methods and hardware.
In well logging and monitoring while drilling NMR applications, however, NMR spectrometers necessarily operate with multiple and lower frequencies and must be functional within a wide range of temperatures that immediately results in less stable antenna and attenuator electrical parameters. Practically, this can be resolved if the attenuator has been designed as frequency independent, i.e., as required no re-tuning.
Typically, in the transmit mode the antenna has been driven by the above mentioned voltage source electronics with an output frequency controlled by crystal oscillators. By periodical compensation of the tank electrical losses a driver maintains a hard synchronization of the antenna current with its voltage (or a tool clock) and establishes a predefined RF pulse magnitude. This apparently simplifies requirements to the antenna stability in the transmit mode.
There are two difficulties encountered in employing a hard synchronization for an imperfectly tuned tank. The first one is associated with elevated tool power consumption, that is, the further apart the driver""s synch and tank""s resonance frequencies, the lower actual tank""s electrical quality, its effective impedance and higher the driver output current.
The second problem appears in the receiver mode where the formation response has a frequency exactly equal to the driver""s frequency response. Thus, if the antenna tuning in the receiver mode does not exactly match the synch, the induced voltage will be amplified less than in Q times. This voltage also will experience the LC tank""s amplitude and phase frequency distortions, which lower the overall SNR. Typically, in well logging and measurement while drilling applications this signal deterioration doesn""t significantly affect the receiver noise figure. Practically, it could be recovered by increasing the NMR train stacking level and possibly with some decreasing of the tool logging speed.
As it has been mentioned above, the most severe problem in NMR well logging apparatus appears when the attenuator has been designed as tunable. Thus, due to the abovementioned temperature influences it becomes difficult to keep attenuator tuning exactly matched to the transmitter antenna driver""s frequency because its parameters change along with changes in temperature and pressure down hole. Changing the attenuator tuning will inevitably reduce its stop-band losses and result in elevating the voltage at the input of receiver circuitry that might exceed a safe level.
Tracking the transmitter synch frequency and sequential real time re-tuning the attenuation device can require parametric reactive elements such, for instance, pre-saturated inductors. To use inductors a biasing field supplied by an external xe2x80x9cexchangeablexe2x80x9d magnet or additional winding carrying a direct current would be required. These, however, would result in unreasonable and often non-achievable stability requirements and introduce additional noise. On other hand, semiconductor voltage controlled capacitors, or varicaps, are impractical due to their low initial capacitance and small voltages that such a device can withstand. However, even if these design constraints were overcome, the attenuator tuning requirements would also deteriorate the NMR tool logging speed that is associated with the time required to accomplish tuning.
Thus, there is a need for a dedicated NMR antenna signal conditioning circuitry that overcomes the limitations discussed above.
The present invention provides an improved and novel active signal conditioning circuitry for NMR transmitter/receiver antenna that is free from the limitations of known NMR antenna designs. Thus, it is an object of the present invention to provide signal conditioning circuitry for connection between a combined NMR transmitter/receiver antenna and the associated receiver circuitry to avoid the problems associated with known NMR transmitter/receiver antennas discussed above. The NMR transmitter/receiver antenna circuitry provided by the present invention comprises electronic circuitry which enables receiver antenna circuit protection from high voltage applied during RF transmitter pulses, provides dumping for the entire antenna in successive stages and eliminates re-tuning during sweeping of the antenna frequency.
Further features and advantages of the invention will become more readily apparent from the following detailed description, when taken in conjunction with the accompanying drawings.